Le 06/06/2015 10:29, Michael Below a écrit :
Am Samstag, den 06.06.2015, 00:01 -0400 schrieb Chris Siebenmann:
In theory I should be able to fiddle with the applied base curve to
tame the high end. In practice the interface of the base curve makes
it extremely difficult to do this, because all of the highlights are
crammed in at the top left and need extremely fine manipulation to get
natural results that recover highlights. Turning off the entire base
curve 'curse' highlights at the expense of everything else, which is
not really an easy way to go.
I disable the base curve with most images. As I understand it, the fact
that the base curve comes early in the processing pipeline means that
you are losing highlights early, befor you can do stuff to differentiate
the tones better. Getting them back after they have been crushed by the
base curve is IMHO more difficult.
I think this is an excellent summary, which explains why Chris has so
much trouble.
The actual topic here is that the dynamic range of the scene is high,
and base curve adjusts by squashing the highlights strongly. Indeed, I
have read that adjusting for limitations (including dynamic range) of
the final display medium should come last in the pipeline (e.g. it's
written in Drago et al's paper available there in PDF: Adaptive
Logarithmic Mapping For Displaying High Contrast Scenes
<http://resources.mpi-inf.mpg.de/tmo/logmap/>).
So, I also disable the base curve most of the time, though not always.
The higher the dynamic range (assuming they are in a RAW file properly
exposed to the right), the darker the image will appear without a base
curve.
When the base curve is turned off, I use the tone curve for contrast,
and a bit of equalizer for the details.
I tried tone curve, too, though it requires more or less manual
adjustment depending on the subject, and it often has effects on
saturation that make difficult to get right color.
Sometimes I used a gentler dedicated base curve that squashes less of
highlights (e.g. 1 EV or 2).
**Chris, since this is a dynamic range topic, have you tried the "global
tonemap" module?**
In practice I mostly use it with the drago operator (disable base curve
first!). Warning: if badly used, it can wreak havoc the image (bad
luminosity, saturation, color "feel"), so use it right.
To get good result in practice I do this:
(1) adjust upstream exposition (first with mouse wheel in the histogram
then finetune if needed in module), which drago actually maps only to
color saturation (seems strange initially but makes sense)
(2) if some areas are still digitally clipped, adjust the "target" to
get them back
(3) adjust overall perceptual luminosity with the "bias".
Sometimes, fine postprocess adjustments are needed using the zone curve,
but they are much easier and with less side effects because the curve is
much closer to a straight diagonal.
Keeping the original image or a reference image displayed in the
snapshot tool make help to get perceived saturation and luminosity right.
With that steps, I obtained more natural-looking pictures than the JPG
made by camera, even under hard sun.
Regards,
--
Stéphane Gourichon
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